United States Patent 19
`Ishii et al.
`
`54 FILE COMPRESSION PROCESSOR
`MONITORNG CURRENT AWALABLE
`CAPACITY AND THRESHOLD WALUE
`
`75
`
`Inventors: Masanori Ishii; Masashi Omuro, both
`of Tokyo, Japan
`73) Assignee: NEC Corporation, Tokyo, Japan
`
`21 Appl. No.: 669,847
`(22 Filed:
`Jun. 26, 1996
`Related U.S. Application Data
`63 Continuation of Ser. No. 139,030, Oct. 21, 1993, abandoned.
`30
`Foreign Application Priority Data
`Oct. 22, 1992
`JP
`Japan .................................... 4-308173
`(51
`int. Clar. G06F 17/30
`52 U.S. Cl. ...............
`... 395/620: 364/715.02
`58 Field of Search ..................................... 395/620, 444,
`395/445, 463,460, 612, 616; 364/715.02
`References Cited
`U.S. PATENT DOCUMENTS
`4,888,681 12/1989 Bams et al. ........................... 395/600
`5,109,433 4/1992. Notemboom .............................. 382fO
`5,128,776 7/1992 Scorse et al. ........................... 358/.426
`5,276,898
`1/1994 Keil et al. ............................... 395/800
`5,313,631
`5/1994 Kao ......................................... 395/600
`5,317,728 5/1994 Truiset al. ............................. 395/600
`5,367,698 11/1994 Webber et al. ......................... 395/800
`5,374,916 12/1994 Chu ................
`... 340/146.2
`5,403,639 4/1995 Belsan et al. ........................... 395/620
`5,579,507
`1/1996 Hosoouchi et al. ............... 395,497.O2
`
`56
`
`USOO5675789A
`Patent Number:
`11
`45 Date of Patent:
`
`5,675,789
`Oct. 7, 1997
`
`OTHER PUBLICATIONS
`Israel et al., “Evolutionary Path to Network Storage Man
`agement", USENIX, 1991 (Winter) p. 185-198.
`Alsberg, "Space and TimeSavings Through Large Path Base
`Compression and Dynamic Restructuring”. Aug. 1975 pp.
`1114-1122, Proceedings of the IEEE, vol. 63 No. 8.
`Lelewer, D.A. et al., "Data Compression", ACM Computing
`Surveys, vol. 19, No. 3, pp. 261-296, (Sep. 1987).
`Primary Examiner-Thomas G. Black
`Assistant Examiner-Jack M. Choules
`Attorney, Agent, or Firm-Foley & Lardner
`57
`ABSTRACT
`A file compression processor of the present invention com
`prises a file status monitor which compares the current
`available capacity on the file unit to record files and the
`threshold value predetermined as the upper limit of the
`available capacity and a file compression portion which
`compresses a file on said file unit when the file status
`monitor judges that the current available capacity is smaller
`than the threshold. It further comprises a file attribute
`controller which stores the control information for the files
`recorded on the file unit including the last access date, the
`number of accesses, whether the file is already compressed
`or not and a search portion which searches for files not yet
`compressed with low access frequency based on the control
`information so as to select files to be compressed for
`compression by said compression portion and a compression
`method selector which determines the data type and the
`access frequency of the file for compression by said com
`pression portion based on the control information, selects
`the file compression method suitable for said data type and
`the access frequency and instructs said compression portion
`to execute processing by the selected compression method.
`15 Claims, 9 Drawing Sheets
`
`
`
`
`
`
`
`
`
`
`
`FESTAS NOR
`PoRo
`
`120
`
`FLE COPRESSION
`PORTION
`
`FEDECOPRESSON
`PORTION
`
`
`
`ATRE
`CONRLLER
`
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`

`Sheet 1 of 9
`
`5,67 5.7 89
`
`U.S. Patent
`
`Oct. 7, 1997
`FIG.1
`100
`J.---------
`- FLE COMPRESSION PROCESSOR:
`
`FILE STATUS MONTOR
`PORTION
`
`
`
`
`
`120
`
`USER PROGRAM
`
`102
`
`105
`
`
`
`FLE COMPRESSION
`PORTION
`
`FLE DECOMPRESSION
`PORTION
`
`104
`
`ATTRIBUTE
`CONTROLER
`
`
`
`
`
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`Sheet 2 of 9
`
`5,675,789
`
`U.S. Patent
`
`Oct. 7, 1997
`FIG.2
`
`START
`
`MONITOR OF AVALABLE FLE
`CAPACITY IN FILE UNIT
`
`21 O.
`
`211
`
`IS AWALABLE FILE CAPACITY
`a
`N FILE UNIT BELOW THRESHOLD
`
`NO
`
`YES
`
`SEARCH FOR FILE TO
`BE COMPRESSED
`
`22O
`
`
`
`ANY FELE TO BE
`COMPRESSED 2
`
`22
`
`NO
`
`
`
`SELECT OF FILE
`COMPRESSION METHOD
`
`
`
`COMPRESSION OF FILE
`TO BE COMPRESSED
`
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`U.S. Patent
`
`Oct. 7, 1997
`
`Sheet 3 of 9
`
`5,675,789
`
`FIG.3
`
`310
`
`
`
`
`
`CHECK OF COMPRESSION METHOD
`FOR COMPRESSION FILE
`
`
`
`DECOMPRESSION OF
`COMPRESSED FILE
`
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`

`U.S. Patent
`
`Oct. 7, 1997
`
`Sheet 4 of 9
`
`5,675,789
`
`100
`
`FitEcoPRESSION PROCESSOR
`
`FILE STATUS MONITOR
`PORTION
`
`101
`
`102
`
`FLE SEARCH
`PORTION
`103
`
`THRESHOLD CHANGE
`PORTION
`
`
`
`
`
`105
`
`FLE COMPRESSION
`PORON
`
`USER PROGRAM
`
`
`
`FILE DECOMPRESSION
`PORTION
`
`FLE
`ATTRIBUTE
`CONTROLLER
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
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`U.S. Patent
`
`Oct. 7, 1997
`
`Sheet 5 of 9
`
`5,675,789
`
`FIG.5
`
`(6)
`
`MONITOR OF AVAILABLE FILE
`CAPACY IN FLE UNIT
`
`2 O
`
`211
`IS AVAILABLE FILE CAPACITY -
`FILE UNIT BELOW THRESHOLD
`
`NO G)
`
`YES
`
`SEARCH FOR FILE TO
`BE COMPRESSED
`
`22O
`
`ANY FILE TO BE
`COMPRESSED
`
`IS THRESHOD
`ADJUSTABLE 2
`
`221
`
`222
`NO
`
`
`
`REDUCTION OF THRESHOLD WALUE
`ACCORDING TO ADJUSTMEN
`
`
`
`
`
`
`
`
`
`m
`
`SELECT OF FILE
`COMPRESSION METHOD
`
`COMPRESSION OF FILE TO
`BE COMPRESSED
`
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`U.S. Patent
`
`Oct. 7, 1997
`
`Sheet 6 of 9
`
`5,675,789
`
`FIG.6
`
`23O
`ISAVAILABLE FILE ?cPCity
`NFILE UNIT OVER INITAL -
`NTHRESHOLD 2-1
`YES
`231
`
`<IAL THRESHOLD VALUED --GC)
`
`YES
`
`NO
`
`RETURNING OF THRESHOLD TO
`INTIAL VALUE
`
`232
`
`
`
`
`
`
`
`
`
`S THRESHOLD
`ADJUSTABLE
`
`233
`NO
`
`NCREASE OF THRESHOLD
`
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`U.S. Patent
`
`Oct. 7, 1997
`
`Sheet 7 of 9
`
`5,675,789
`
`FIG.7
`
`235
`Is DFFERENCE BETWEEN AVAILABLE FILE
`APACITY AND THRESHOLD LARGER THA
`MNIMUM DIFFERENCE BETWEEN FL
`SIZES BEFORE AND AFTER COMPRESSION ?
`
`
`
`SEARCH FOR COMPRESSED FILE
`
`
`
`
`
`
`
`ANY COMPRESSED FILE 2
`
`DECOMPRESSION AND
`DECOMPRESSION OF
`COMPRESSED FLE
`
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`

`U.S. Patent
`
`Oct. 7, 1997
`
`Sheet 8 of 9
`
`5,675,789
`
`FIG.8
`
`X
`
`M
`
`
`
`THRESHOLD
`
`AVAILABLE CAPACTY
`THRESHOLD
`AFTER ADJUSTMENT
`
`FIG.9
`
`RETURNING
`TO INTAL VALUE
`
`AVAILABLE CAPACTY
`
`INTAL THRESHOLD
`
`CURRENT THRESHOLD
`
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`U.S. Patent
`
`Oct. 7, 1997
`
`Sheet 9 of 9
`
`5,675,789
`
`FIG.10
`
`was up
`
`a v0 se p are no
`
`a
`
`a
`
`a
`
`h
`
`is
`
`as
`
`INTAL THRESHOLD
`
`NCREASE
`BY ADJUSTMENT
`Xn
`VALUE Xn
`
`AVAILABLE CAPACITY
`
`THRESHOD
`-
`AFTER MODFICATION
`
`CURRENT THRESHOLD
`
`FG.11
`
`DECOPRESSION AND
`DECOPRESSION OF
`CONPRESSED FLE
`
`
`
`
`
`
`
`
`
`
`
`CURRENT AWALABLE CAPACITY
`
`AVAILABLE CAPACITY
`AFTER MODIFICATION
`- INITIAL THRESHOLD
`(=CURRENT THRESHOLD)
`
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`

`5,675,789
`
`1.
`FILE COMPRESSION PROCESSOR
`MONTORING CURRENT AWALABLE
`CAPACITY AND THRESHOLD WALUE
`
`2
`compares the current available capacity on a file unit to
`record files and the threshold value predetermined as the
`upper limit of the available capacity value and file compres
`Sion means which compresses a file on the file unit when the
`file status monitor means judges that the current available
`capacity is smaller than the threshold.
`According to the still preferred construction, a file com
`pression processor further comprises a file attribute control-,
`ler which stores the control information for the files recorded
`on the file unit including the last access date, the number of
`accesses and whether the file is already compressed or not
`and file search means which searches for files not yet
`compressed with low access frequency based on the control
`information of the file attribute controller so as to selectfiles
`to be compressed for compression by the compression
`CS.
`According to the still preferred construction, a file com
`pression processor further comprises a file attribute control
`ler which stores the control information for the files recorded
`on the file unit. The control information includes the last
`access date, the number of accesses, whether the file is
`already compressed or not and the data type. Also provided
`is a compression method selection means which determines
`the data type and the access frequency of the file compressed
`by the compression means based on the control information
`of the file attribute controller, selects the file compression
`method suitable for the data type and the access frequency
`and instructs the compression means to execute processing
`by the selected compression method.
`According to another preferred construction, a file com
`pression processor further comprises a file attribute control
`ler which stores the control information for the files recorded
`on the file. The control information includes the last access
`date, the number of accesses, whether the file is already
`compressed or not and the data type. Also included is a file
`search means which searches for files not yet compressed
`with a low access frequency based on the control informa
`tion of the file attribute controller, so as to select files to be
`compressed for compression by the compression means.
`Further, there is a compression method selection means
`which determines the data type and the access frequency of
`the file to be compressed selected by the file search means
`based on the control information of the file attribute
`controller, selects the file compression method suitable for
`the data type and the access frequency and instructs the
`compression means to execute processing by the selected
`compression method.
`According to another preferred construction, a file com
`pression controller further comprises file decompression
`means which decompresses files already compressed corre
`sponding to the access request from the compression file
`recorded on the file unit.
`According to another aspect of the present invention to
`attain the above objects, a file compression processor com
`prises a file status monitor means which compares the
`current available capacity on afile unit to record files and the
`threshold value predetermined as the upper limit of the
`available capacity value. There is a file compression means
`which compresses a file on the file unit when the file status
`monitor means judges that the current available capacity is
`Smaller than the threshold. Also, there is a threshold change
`means which reduces the threshold by the predetermined
`adjustment value when the current available capacity is
`Smaller than the threshold and there is no file to be
`compressed, and which increases the threshold value when
`the current available capacity is larger than the threshold
`value.
`
`This application is a continuation of application Ser. No.
`08/139,030, filed Oct. 21, 1993, now abandoned.
`BACKGROUND OF THE INVENTION
`1. Field of the Invention
`The present invention relates to a file compression pro
`cessor more particularly, it relates to a file compression
`processor which records image data and/or text data to file
`recording media such as magneto-optic disks (MO) and hard
`disks after data compression processing.
`2. Description of the Related Art
`Recently, on-line information services have become wide
`spread. In such services, a database is created by recording
`text data and binary data (programs and image data) on a
`large capacity storage media such as magneto-optic disks
`and hard disks, so that a user can access such a database
`from a terminal through communication lines when neces
`sary and can retrieve the desired data.
`Such databases usually have an extremely large volume of
`data and utilize various data compression techniques for
`recording data to the storage media, which increase the
`virtual capacity of the record files.
`Examples of such data file compression technology in
`such a system are disclosed by Data Compression, ACM
`Computing Surveys, Vol. 19, No. 3 pp. 261-296. September
`1987.
`In a conventional data file compression system using such
`a data compression method, all files are compressed for
`storage to a file unit such as disks. This means that the files
`are compressed even when the file unit has sufficient avail
`able capacity. Decompression processing is indispensable to
`use such files, which results in reduced efficiency of decom
`pression processing.
`Further, the data compression method is limited to only
`one. A different compression method cannot be used even
`when the applicable file does not require the same
`compression/decompression speed and compression ratio,
`depending on the access frequency and the file type deter
`mined by the last access data and the number of accesses of
`the file to be compressed.
`SUMMARY OF THE INVENTION
`The present invention is made to solve the above draw
`backs of the prior art. It is a first object of the present
`invention is to provide a file compression processor which
`enables effective file utilization and prevents file compres
`sion when there is sufficient available capacity in the file unit
`and selects the suitable compression method corresponding
`to the access frequency and the type of the file.
`It is a second object of the present invention to provide a
`file compression processor for effective file utilization by
`enabling increase/decrease of the threshold serving as the
`reference of the available file capacity (the capacity to be
`always ensured).
`It is a third object of the present invention to provide a file
`compression processor which enables file utilization with
`minimum decompression by decompressing these com
`pressed files with high access frequency when the available
`file capacity increased. According to one aspect of the
`present invention to attain the above objects, a file compres
`sion processor comprises file status monitor means which
`
`5
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`O
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`5
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`

`5,675,789
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`15
`
`20
`
`3
`According to another preferred construction, a file com
`pression processor further comprises a file attribute control
`ler which stores the control information for the files recorded
`on the file including. The control information includes the
`last access date, the number of accesses, whether the file is
`already compressed or not and the data type. Also, a file
`search means searches for files not yet compressed with low
`access frequency, based on the control information of the file
`attribute controller, so as to select files to be compressed for
`compression by the compression means.
`10
`According to another preferred construction, a file com
`pression processor further comprises a file attribute control
`ler which stores the control information for the files recorded
`on the file unit including the last access date, the number of
`accesses, whether the file is already compressed or not and
`the data type. There is a compression method selection
`means which determines the data type and the access
`frequency of the file compressed by the compression means,
`based on the control information of the file attribute
`controller, which selects the file compression method suit
`able for the data type and the access frequency and which
`instructs the compression means to execute processing by
`the selected compression method.
`According to another preferred construction, a file com
`25
`pression processor further comprises a file attribute control
`ler which stores the control information for the files recorded
`on the file unit. The control information includes the last
`access date, the number of accesses, whether the file is
`already compressed or not and the data type. There is a file
`search means which searches for files not yet compressed,
`with low access frequency based on the control information
`of the file attribute controller, so as to select files to be
`compressed for compression by the compression means.
`There is also a compression method selection means which
`determines the data type and the access frequency of the file
`to be compressed selected by the file search means, based on
`the control information of the file attribute controller, which
`selects the file compression method suitable for the data type
`and the access frequency and which instructs the compres
`Sion means to execute processing by the selected compres
`sion method.
`According to another preferred construction, a file com
`pression processor further comprises a file attribute control
`ler which stores the control information for the files recorded
`on the file unit. The control information includes the last
`access date, the number of accesses, whether the file is
`already compressed or not, the maximum and the minimum
`differences between the file sizes before and after compres
`sion. There is a file search means which searches for files not
`yet compressed, with low access frequency based on the
`control information of the file attribute controller, so as to
`select files to be compressed for compression by the com
`pression means, the threshold change means reducing the
`threshold by the predetermined adjustment value if there is
`no file to be compressed and the threshold value becomes
`larger than the maximum difference between the file sizes
`before and after compression when the threshold value is
`reduced by the predetermined adjustment value.
`According to the still preferred construction, a file com
`pression processor further comprises a file decompression
`means which decompresses files already compressed corre
`sponding to the access request from the compression file
`recorded on the file unit.
`According to another preferred construction, a file com
`65
`pression processor further comprises a file decompression
`means which decompressed decompresses compressed files
`
`45
`
`4
`until the available capacity becomes close to the initial
`value, when the file status monitor means judges that the
`threshold is the initial value and the available capacity is
`larger than the threshold.
`According to another preferred construction, a file com
`pression processor further comprises a file attribute control
`ler which stores the control information for the files recorded
`on the file unit. The control information includes the last
`access date, the number of accesses, whether the file is
`already compressed or not, the maximum and the minimum
`differences between the file sizes before and after compres
`sion. There is a file search means which searches for already
`compressed files with high access frequency, based on the
`control information of the file attribute controller, so as to
`select compressed files for decompression by the decom
`pression means.
`According to another preferred construction of a file
`compression processor, the compressed files are selected for
`decompression by the decompression means when it is
`judged that the threshold is the initial value and the differ
`ence between the available capacity and the threshold is
`larger than the minimum difference between the file sizes
`before and after compression.
`According to another preferred construction of a file
`compression processor, the threshold change means returns
`the threshold to the initial value when it judges that the
`threshold is not the initial value and the available capacity is
`larger than the initial thershold value.
`According to another preferred construction of a file
`compression processor, the threshold change means
`increases the threshold by the product of the predetermined
`adjustment value and the integer obtained by dividing the
`difference between the available capacity and the threshold
`with the predetermined adjustment value, when it is judged
`that the threshold is not the initial value and the available
`capacity is Smaller than the initial threshold value, and the
`difference between the available capacity and the threshold
`is larger than the predetermined adjustment value.
`Other objects, features and advantages of the present
`invention will become clear from the detailed description
`given herebelow.
`BRIEF DESCRIPTION OF THE DRAWINGS
`The present invention will be understood more fully from
`the detailed description given herebelow and from the
`accompanying drawings of the preferred embodiment of the
`invention, which, however, should not be taken to be limi
`tative as to the invention, but are for explanation and
`understanding only.
`In the drawings:
`FIG. 1 is a block diagram of a file compression processor
`according to a first embodiment of the present invention;
`FIG. 2 is a flowchart to show the operation of file
`compression processing of the file compression processor
`according to the first embodiment;
`FIG. 3 is a flowchart to show the access operation by user
`program according to the first embodiment;
`FIG. 4 is a block diagram of a file compression processor
`according to a second embodiment of the present invention;
`FIG. 5 is a flowchart to show the operation of the file
`compression processor according to the second embodi
`ment;
`FIG. 6 is a flowchart to show the operation of the file
`compression processor according to the second embodi
`ment,
`
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`5,675,789
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`O
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`5
`
`5
`FIG. 7 is a flowchart to show the operation of the file
`compression processor according to the second embodi
`ment,
`FIG. 8 is an explanatory view to illustrate the reduction of
`the threshold in Steps 222 and 240 of FIG. 5;
`FIG. 9 is an explanatory view to illustrate resetting to the
`initial threshold value in FIG. 6;
`FIG. 10 is an explanatory view to illustrate increasing of
`the threshold value in FIG. 6; and
`FIG. 11 is an explanatory view to illustrate the decom
`pression processing for "compressed files” in Steps 235 to
`238 in FIG. 7.
`DETAILED DESCRIPTION OF PREFERRED
`EMBODIMENT
`Referring to the attached figures, preferred embodiments
`of the present invention will be described below. FIG. 1 is
`a block diagram of a file processing System using a file
`compression processor according to a first embodiment of
`the present invention. In the figure, a file processing system
`comprises a file compression processor 100, a user program
`120 and a file unit 130.
`The file compression processor 100 serves for automatic
`file compression and decompression processing asynchro
`nously with the user program 120 for a disk 131 in the file
`unit 130. To use a file, the user program 120 accesses a file
`recorded on the file unit 130 via the file compression
`processor 100. The file unit 130 comprises disks 131 which
`are recording media such as magneto-optic disks (MO) or
`hard disks, and a file attribute controller 132 to control
`information on the files recorded on the disks 131.
`The file compression processor 100 comprises, as shown
`in the figure, a file status monitor portion 101, a file search
`portion 102, a file compression method selection portion
`104, afile compression portion 105 and a file decompression
`portion 106.
`The file status monitor portion 101 judges whether the
`current available file capacity on the file unit 130 controlled
`by the file attribute controller 132 is over or below the
`predetermined threshold value (the available file capacity to
`be ensured in the file unit 130).
`When the file status monitor portion 101 judges that the
`current available file capacity is below the threshold, the file
`search portion 102 searches the files to be compressed (all
`files except those for which compressionis prohibited by the
`user) for a file not yet compressed with a lower access
`frequency. The search of files to be compressed is made by
`referring to the file control information at the file attribute
`controller 32.
`The file attribute controller 132 has as file control
`information, including a list of file names and directories, as
`well as the last access date and the number of accesses for
`each file. The access frequency of a file is determined based
`on the last access date and the number of accesses for that
`file and is given as high, medium and low. The standard of
`access frequency classification is set in the file search
`portion 102. Alternatively, a user may set the standard or
`classes.
`The file compression portion 105 actually compresses
`data of the searched file to be compressed, so as to record it
`on the disk 131 in the file unit 130. According to the
`attributes of the file to be compressed including access
`frequency and file type, it selects an appropriate data com
`pression method for compression of the file.
`65
`The file compression method selection portion 104 selects
`the method used by the data compression portion 105 to
`
`40
`
`6
`compress data of the file to be compressed. It selects the data
`compression method suitable for the file considering the data
`attribute (whether it is text data or binary data such as
`programs and image data) and access frequency, included
`among the control information concerning the file in the file
`attribute controller 132.
`The data attribute of a file is checked in the selection of
`the data compression method so as to select a suitable data
`compression method with a higher compression ratio. The
`access frequency of a file is used for selecting a data
`compression method with shorter data compression?
`decompression time. The data compression methods selec
`tion portion 104 selects, from some data compression meth
`ods selected according to the data attribute, the one with
`shorter data compression/decompression time for a file with
`higher access frequency and the one with a higher compres
`sion ratio for a file with lower access frequency.
`The file attribute controller 132 of the file unit 130
`controls, as described above, the available file capacity of
`the file unit 130, file names and directory names, the
`maximum and minimum differences between the file sizes
`before and after the file compression, and names of the files
`with such maximum and minimum values, as well as infor
`mation for each file including file size, data attribute (text
`data file, program file or image data file), identifier of
`compression prohibition, whether the file is already com
`pressed or not yet compressed (compression/non
`compression information), compression method, the last
`access date, the number of accesses, and file size after
`compression (for already compressed files). The indicator of
`compression prohibition is given to the file for which the
`user specified compression prohibition.
`Though the file attribute controller 132 is physically
`separated from the disks 131 in FIG. 1, the files actually
`have a file control information area to record the above file
`control information separately from the data area, and the
`file control information in such file control information area
`is controlled by the file attribute controller 132.
`The file decompression portion 106 serves for decom
`pression of a file in response to a file utilization request from
`the user program 120. It refers to the data compression
`method as one of the file control information so as to
`perform decompression processing. However, if the file is
`found to be a "non-compression” file as a result of checking
`the compression/non-compression information at the file
`attribute controller 132, decompression is not made at all
`and the file is just read out from the file unit 130.
`FIG. 2 is a flowchart to illustrate the operation of the
`system shown in FIG.1. The file status monitor portion 101
`monitors the current available file capacity in the file unit
`130 asynchronously with the user program 120 (Step 210).
`Available capacity and available file area information for the
`file unit 130 are stored in the file attribute controller 132 on
`the file unit 130 as file control information and such infor
`mation is controlled by the file control function of the
`operating system. Upon any change in available file area or
`available capacity information, the file control information
`in the file attribute controller 132 is updated, and creation,
`deletion, extension and reduction of a file causes the infor
`mation to be changed under the control of the operation
`system.
`The file status monitor portion 101 compares the current
`available file capacity and the threshold at the file unit 130
`(Step 211). If the available file capacity is larger than the
`threshold, the data compression is not made because the file
`unit 130 has sufficient available file capacity. Since the data
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`is not compressed in this case, the file can be used without
`decompression processing immediately after a file utiliza
`tion request. This results in speedy file accessing.
`It is not until the available capacity becomes lower than
`the threshold value that files are compressed. For such
`compression, a search for files to be compressed is con
`ducted based on the file control information of the file
`attribute controller 132 (Step 220). From the files other than
`those for which compression is prohibited by the user,
`non-compression files with low access frequency are picked
`up. If there is no file to be compressed, the system returns to
`Step 210. In Step 221, it is judged whether there is any file
`to be compressed. The term "File to be compressed” means
`the files subject to compression until the available file
`capacity exceeds the predetermined threshold value.
`When files to be compressed are picked up, they are
`subject to data compression processing (Step 250). The file
`compression method selection portion 104 here selects the
`method with suitable compression ratio and compression?
`decompression speed depending on the file access frequency
`and data attribute.
`In selecting a data compression method, the file compres
`sion method selection portion 104 selects the one suitable
`for the applicable data attribute by checking the data
`attribute held by the file attribute controller 132. Next, the
`access frequency is determined from the last access date and
`the number of accesses at the file attribute controller 132.
`The file compression method with a shorter decompression
`time is selected for files with higher access frequency and
`the file compression method with a higher compression ratio
`is selected for files with lower access frequency. The data
`compression methods used here are known methods. For
`example, the following data compression methods may be
`utilized in this embodiment.
`Data compression methods for text files include Huffman
`coding, arithmetic coding and Lempel–Ziv coding. In Huff
`man coding, the number of bits to represent a character is
`changed depending on the frequency of the character. Huff
`man coding involves a code table in tree structure, which
`may be fixed or variable. The former is called the static
`Huffman coding and the latter the dynamic Huffman coding.
`Arithmetic coding is the method where Huffman coding is
`applied to periods considering the probability of appearance
`for the character in the period. Lempel–Ziv coding is the
`method taking advantage of repetition of character strings,
`where compression is made by deleting redundant strings. In
`general, Lempel–Ziv coding has a lower compression ratio
`than Huffman and arithmetic coding but has a higher
`compression/decompression speed. The file compression
`method selection portion 104 selects Lempel–Ziv coding for
`files with high access frequency and Huffman or arithmetic
`coding for files with low access frequency.
`Data compression methods for binary files (image data)
`include the run-length coding and the differential mapping
`method. The former represents the consecutive identical
`data by the number. By representing the pixels in an image
`with numbers, it can largely save the number of bits for the
`number of strings of image elements, but it has a lower
`processing speed. The difference mapping method repre
`sents the image not by the brightness values themselves but
`by the difference from the adjacent pixel brightness given in
`an array. The file compression method selection portion 104
`selects, in the case of image data, the differential mapping
`method for files with high access frequency and the run
`length coding for files with low access frequency.
`Files are compressed by the data compression method
`thus selected (Step 260). Upon completion of file compres
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`sion processing, the system returns to Step 210 to repeat the
`procedure. The file compression processing is repeated until
`the available file capacity becomes higher than the threshold
`value. When it is judged that the available file capacity is
`over the threshold in Step 211, the following compression
`processing